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Cancer magisterDungeness crab

By Larissa Khatain

Geographic Range

Cancer magister, commonly known as Dungeness crab, is found in the costal waters from Point Concepcion, California, to the Pribilof Islands, Alaska. The Dungeness crabs inhabit the estuaries from Morro Bay, California, to Puget Sound, Washington. (California Dept. of Fish and Game, 1994)

Habitat

Dungeness crabs are found on the Pacific coast in sandy bottoms below the tidal mark. They can also be found at lowtide in sandy or muddy bays where there is a good growth of eel grass. Dungeness crabs are intolerant of low dissolved oxygen conditions. Also, even low amounts of ammonia are toxic to the crabs. Dungeness crabs also tend to grow better in water that is above six degrees Celsius (Kozloff 1973).

Physical Description

Cancer magister is a decapod. Decopods' thoracic segment is fused with that of the head, to form the cephalothorax, which is covered by a carapace. The Dungeness crab has five pairs of thoracic legs. The first pair of legs is larger then the last four and has pinching claws. Cancer magister is a walking crab and therefore, the last pair of legs is adapted for walking. It has a flat and broad body, oval in shape. The anterior margin of the carapace has nine small teeth on each side, forming an elliptical curve. At the end of the curve, a large, pointed tooth projects directly outward. From this tooth, the carapace slopes backward, forming a narrow posterior end. Males range from 18 to 23 centimeters (about 7 to 9 inches)in width and 10 to 13 centimeters (4 to 5 inches) long. The color of the carapace is reddish-brown, fading towards the back. The legs and ventral side are yellowish. (Headstrom, 1979)

  • Range length
    10 to 13 mm
    0.39 to 0.51 in

Reproduction

Males attract the females by use of pheromones, which are chemical scents. During mating, the male crabs clasp the female so that the undersides of each are close. The male's breeding structures place the sperm into the female's body. This is only possible when the female is soft shelled, right after molting. This lasts less than thirty minutes. Mating occurs in near-shore costal locations, outside of estuaries. The eggs are not fertilized and spawned until the fall, following the summer breeding. After fertilization, about half a million to one million eggs are attatched to the female's abdomen. These eggs are brooded there until spring. The larvae are planktonic and use tidal currents to travel into estuaries. The larvae pass through six stages over a 105 to 125 day period. The last two stages are zoea and megalopa. Zoea have a jointed abdomen and a spined carapace behind the head with large eyes. Megalops have big eyes, an extended abdomen, elongated carapace and swimming legs. After the first molting, the form changes to that similar to the adults. Growth after this point occurs by shedding its shell, molting, at certain periods of time, until it reahces full growth (Headstrom 1979, Mash 1975).

Behavior

Dungeness crabs bury themselves almost completely with sand. While covered, they are able to keep from suffocating due to hairs located above water intakes at the bases of their claws. These hairs keep the gill chamber free of sand grains.

The crab finds its prey by probing its sensitive claws into the sand. The crab then proceeds to eat the prey by using its claws to tear apart the food. Then, using its smaller feeding appendages, it passes the food to its mouth. Here it is crushed by two hard mandibles.

Dungeness crabs can walk in all directions, not just sideways. Occasionally, Dungeness crabs can run quickly (StreamNet 2001).

Dungeness crab are preyed upon in all stags of their lives. Early in life, marine worms eat their eggs. Larvae are an important food for Pacific herring, Pacific sardines, rockfish, and chinook salmon. Juvenile crabs fall prey to starry flounder, rock sole, lingcod, rockfish, sturgeon, sharks, and skates. As adults, Dungeness crabs are eaten by humans, harbor seals and sea lions (Streamnet 2001). (StreamNet, 2001)

Food Habits

Cancer magister eat a variety of marine invertebrates and fish. As juveniles, the Dungeness crabs feed on fish, shrimp, molluscs and crustaceans. Adults feed on bivalves, crustaceans and fishes. The crabs are able to open shells by chipping away at them with their heavy pinching claws. (California Dept. of Fish and Game, 1994)

Predation

Seals, sea lions, and a variety of fish eat Dungeness crabs. (Batis and Kaelin, 2000; California Dept. of Fish and Game, 1994; StreamNet, 2001)

Economic Importance for Humans: Positive

Dungeness carbs are an important commercial shellfish. Male crabs are harvested along the coast of North America from Alaska to California. The fishery is worth tens of millions of dollars, due to the thousands of crabs caught annually. (Batis and Kaelin, 2000; California Dept. of Fish and Game, 1994)

  • Positive Impacts
  • food

Conservation Status

Dungeness crabs are affected by many insecticides. The insecticide carbaryl, also known as Sevin, is particularly toxic to the Dungeness crab. Other toxins include other insecticides and fungicides as well as ammonia. Urban pollutants such as heavy metals, PCB's and hydrocarbons also affect the Dungeness crab. Runoff of pesticides and herbicides affect the Dungeness crab populations as well. Dungeness crabs are not endangered, however, these chemicals can kill or upset the health of Dungeness crab populations. (StreamNet, 2001)

Other Comments

Dungeness crab were named after a fishing town on the coast of Washington. Only the males are harvested, the females are thrown back into the water.

A tagging study was conducted by the California Department of Fish and Game. This study showed that crabs reared in the San Francisco Bay grew about twice the rate as ocean-reared crabs. The average carapace was appoximately 100 millimeters after one year. The hypothesis is that warmer bay temperatures and increased food availability is resposible for the rapid growth. (California Dept. of Fish and Game, 1994)

Contributors

George Hammond (editor), Animal Diversity Web.

Larissa Khatain (author), Southwestern University, Stephanie Fabritius (editor), Southwestern University.

Glossary

Pacific Ocean

body of water between the southern ocean (above 60 degrees south latitude), Australia, Asia, and the western hemisphere. This is the world's largest ocean, covering about 28% of the world's surface.

World Map

benthic

Referring to an animal that lives on or near the bottom of a body of water. Also an aquatic biome consisting of the ocean bottom below the pelagic and coastal zones. Bottom habitats in the very deepest oceans (below 9000 m) are sometimes referred to as the abyssal zone. see also oceanic vent.

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.

brackish water

areas with salty water, usually in coastal marshes and estuaries.

carnivore

an animal that mainly eats meat

carrion

flesh of dead animals.

coastal

the nearshore aquatic habitats near a coast, or shoreline.

ectothermic

animals which must use heat acquired from the environment and behavioral adaptations to regulate body temperature

estuarine

an area where a freshwater river meets the ocean and tidal influences result in fluctuations in salinity.

food

A substance that provides both nutrients and energy to a living thing.

heterothermic

having a body temperature that fluctuates with that of the immediate environment; having no mechanism or a poorly developed mechanism for regulating internal body temperature.

insectivore

An animal that eats mainly insects or spiders.

motile

having the capacity to move from one place to another.

native range

the area in which the animal is naturally found, the region in which it is endemic.

saltwater or marine

mainly lives in oceans, seas, or other bodies of salt water.

solitary

lives alone

temperate

that region of the Earth between 23.5 degrees North and 60 degrees North (between the Tropic of Cancer and the Arctic Circle) and between 23.5 degrees South and 60 degrees South (between the Tropic of Capricorn and the Antarctic Circle).

zooplankton

animal constituent of plankton; mainly small crustaceans and fish larvae. (Compare to phytoplankton.)

References

Batis, J., A. Kaelin. 2000. "Dungeness Crab" (On-line). World of Aquaculture. Accessed 10/27/04 at http://www7.taosnet.com/platinum/data/light/species/crabdungeness.html.

California Dept. of Fish and Game, 1994. "Dungeness Crab (Cancer magister)" (On-line). Central Valley Bay-Delta Monitoring. Accessed 10/27/04 at http://www.delta.dfg.ca.gov/baydelta/monitoring/cmag.asp.

Headstrom, R. 1979. Lobsters, Crabs, Shrimps and Their Relatives. New York: A.S. Barnes and Company.

Kozloff, E. 1973. Seashore Life of Puget Sound, the Strait of Georgia, and the San Juan Islands. Seattle: University of Washington Press.

Mash, K. 1975. How Invertebrates Live. New York: E.P. Dutton and Company Inc..

StreamNet, 2001. "Life History Profiles - Fresh Water and Estuary Fish of the Pacific Northwest" (On-line). StreamNet - Fish Data for the Northwest. Accessed 10/27/04 at http://www.streamnet.org/pub-ed/ff/Lifehistory/.

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